Cytokinesis is a crucial step in mitotic cell division which is the basis for the growth and development of eukaryotic organisms as well as for pathological conditions such as cancer. Cytokinesis is also an important mechanism for the generation of embryonic asymmetry. Therefore, understanding the mechanism and regulation of cytokinesis is likely to have broad medical implications. In animal cells, cytokinesis is known to be dependent on an actomyosin-based contractile ring. Our long term goal is to understand the spatial and temporal regulation of contratile ring assembly and function. This knowledge could also facilitate studies of other actomyosin-dependent cellular processes, such as cell locomotion and purse-string wound healing. We have recently identified an actomyosin-based ring structure in budding yeast and demonstrated its critical function during cyokinesis. This finding provides a unique opportunity for studyinh this fundamental process using a combination of genetic, biochemical and video microscopy approaches in yeast. Specifically, Dr. Li will investigate the mechanism of actin and myosin II localization during contractile ring assembly, the regulation of actomyosin ring contraction, as well as the mechanism of ring disassembly during contraction. A second goal is to understand how cytokinesis is coordinated with anaphase spindle dynamics, because such coordination is important for the successful partitioning of genetic material to progeny cells. We have already identified several structural and signaling proteins that play crucial roles in the actomyosin ring activities, and checkpoint proteins that are likely to be important for the coordination of cytokinesis with other anaphase events. We will carry out further genetic and biochemical analysis to determine their mechanism of function. Because most of these proteins have mammalian homologs, our findings should facilitate studies of similar processes in other eukaryotic organisms.